The invention relates to oil shale retorting and more particularly to staged fluidized bed oil shale retorting.
Retorting of oil shale can be produced in a fluidized bed in which raw shale particles are continuously fed into the top of a column, gas is forced up through the bed of small shale particles which behave as a viscous liquid and heat is added to produce pyrolysis. At any particular temperature a fixed time is required for complete pyrolysis so, ideally, each particle should remain in the bed for the same length of time. In an "ideally mixed" fluidized bed the temperature, pressure and composition of the material is the same throughout. The actual residence time, however, even for uniform sized particles, shows a wide distribution about the average time. The use of staging, passing through a series of vessels in sequence, narrows the distribution of residence time; generally 4-6 stages are considered sufficient.
However, particles from a crusher have a wide range of size. For the larger and heavier particles a higher gas velocity is required to fluidize the bed. But if the heavier particles are fluidized, the lighter particles are swept up by the gas flow, introducing a loss. If a lower gas velocity is used, the larger, heavier particles will sink through the fluid so their residence time is shorter. Ideally the residence time is identical for all particles in the bed.
U.S. Pat. No. 4,199,432 to Tamm, et al., issued Apr. 22, 1980 shows a staged turbulent bed retorting process which utilizes a partially fluidized bed which behaves as a serial plurality of stages. By utilizing this configuration to achieve staging, the small particles all have approximately the same residence time. Baffles, e.g., horizontally disposed perforated plates, are placed in the column to slow down the larger unfluidized particles so their residence is about 50-90 percent of the average residence time for all particles passing through the retort.
U.S. Pat. No. 2,717,869 to Turner, issued Sept. 13, 1955 shows a fluidized bed retort with horizontal baffles to prevent shale particles from falling freely through the bed and, thus, to increase the retorting time.
U.S. Pat. No. 4,336,128 to Tamm, issued June 22, 1982 shows a staged turbulent bed process in which the retort column has multiple perforated horizontal plates to serve as dispersing elements. The system also contains a combustion chamber with multiple perforated horizontal plates as dispersing elements to limit backmixing, limit slugging and increase residence time of non-fluidizable coarse particles passing through the chamber. The dispersing elements increase the residence time of non-fluidizable particles to about 50-90 percent of the average residence time for all particles. U.S. Pat. Nos. 4,336,127 to Bertelsen and 4,332,669 to Spars, et al., also show retorts having horizontal barriers, baffles, dispersers or flow redistributers which include spaced horizontal perforated plates, bars, screens, packing or other suitable internals.
U.S. Pat. Nos. 4,145,274 to Green, et al., 4,102,311 to Greene, 3,384,569 to Peet and 2,573,906 to Huff show a variety of processes in which the material goes through a series of separate processing zones.
Accordingly, it is an object of the invention to equalize the residence time for different size particles in a fluidized bed.
It is also an object of the invention to produce a narrow distribution of residence time for particles of any particular size in a fluidized bed.
It is another object of the invention to provide method and apparatus for oil shale retorting in a fluidized bed in which oil shale particles of various sizes remain in the fluidized bed for sufficient time for complete pyrolysis to occur.